Wavelength conversion elements featuring a semiconductor laser can be made smaller and have higher output, and the wavelength can be shortened by utilizing wavelength conversion to a higher harmonic wave. On the other hand, the wavelength tolerance for a wavelength conversion element to convert wavelengths at high efficiency is generally extremely narrow, which means that the oscillation wavelength of the semiconductor laser has to be stabilized to stabilize the output.
Providing optical feedback to the semiconductor laser has been proposed as a way to solve this problem. The waveguide mode of a semiconductor laser can be controlled by external optical feedback. For instance, it has been indicated that the oscillation wavelength of a semiconductor laser can be fixed by using a narrow-band wavelength selecting filter or a fiber grating to subject light emitted from a semiconductor laser to wavelength selection, and then providing feedback to the resonator of the semiconductor laser (see Patent Document 1, for example). There has also been proposed a method for controlling the oscillation wavelength of a semiconductor laser by using an external grating to return light from the outside (see Patent Document 2, for example).
FIG. 8 shows an example of a conventional coherent light source. A band pass filter 504 is used to provide optical feedback to a semiconductor laser 501, and the oscillation wavelength of the semiconductor laser is fixed at the transmission wavelength of the band pass filter 504. A dichroic mirror 505 has the characteristics of completely reflecting higher harmonic waves and transmitting fundamental waves, and the band pass filter 504 is designed to transmit only a fundamental wave of a selected wavelength. A fundamental wave that exits the semiconductor laser 501 is focused by a focusing optical system 502 and is incident on a wavelength conversion element 503. Part of the fundamental wave is converted into a higher harmonic wave by the wavelength conversion element 503, and after passing through a coherent lens, the higher harmonic wave is subjected to wavelength separation by the dichroic mirror 505 and taken off to the outside as a higher harmonic wave. Meanwhile, the fundamental wave emitted from the wavelength conversion element passes through the coherent lens 510, then passes through the dichroic mirror 505, and is selected for a specific wavelength by the band pass filter 504. After this, the fundamental wave is reflected by a mirror 513, goes back along the same path, and is fed back into the active layer of the semiconductor laser 501. Since the power of the feedback wavelength is increased within the active layer of the semiconductor laser 501, there is an apparent decrease in the loss of light of the feedback wavelength within the resonator, so the oscillation wavelength is fixed at the feedback wavelength. Since the transmission wavelength can be controlled by adjusting the angle of the band pass filter 504, it is possible to perform wavelength conversion very efficiently by adjusting the oscillation wavelength of the semiconductor laser to the phase matching wavelength of the wavelength conversion element 503.
Patent Document 1: Japanese Unexamined Patent Publication H10-186427
Patent Document 2: Japanese Unexamined Patent Publication H06-102552